Technical Field
The present disclosure relates to an integrated pressure sensor with double measuring scale, to a pressure measuring device including the integrated pressure sensor, to a braking system, and to a method of measuring a pressure that uses the integrated pressure sensor. In particular, the ensuing treatment will make explicit reference, without this implying any loss of generality, to use of said pressure sensor in a braking system of a vehicle, in particular an electromechanical braking system of the BbW (Brake-by-Wire) type.
Description of the Related Art
As is known, disk-brake systems of a traditional type for vehicles comprise a disk fixed with respect to a respective wheel of the vehicle, a calliper associated with the disk, and a hydraulic control circuit. The calliper houses within it pads of friction material, and one or more pistons connected to the hydraulic control circuit. Following upon an action, exerted by a user of the vehicle, on the brake pedal, a pump in the hydraulic control circuit pressurizes a fluid contained in the circuit itself. Consequently, the pistons, equipped with purposely provided sealing elements, come out of respective seats and come to press the pads against the surface of the disk, in this way exerting a braking action on the wheel.
Recently, so-called DbW (Drive-by-Wire) systems have been proposed, which envisage electronic control of the main functions of a vehicle, for example the steering system, the clutch, and the braking system. In particular, electronically controlled braking systems have been proposed, which envisage replacement of the hydraulic callipers with actuators of an electromechanical type. In detail, appropriate sensors detect operation of the brake pedal and generate corresponding electrical signals, which are received and interpreted by an electronic control unit. The electronic control unit then controls intervention of the electromechanical actuators (for example, pistons driven by an electric motor), which exert the braking action on the corresponding brake disks, through the pads. The electronic control unit further receives from sensors associated to the braking system information on the braking action exerted by the electromechanical actuators so as to provide an appropriate closed-loop feedback control, for example, via a PID (Proportional-Integral-Derivative) controller. In particular, the electronic control unit receives information on the pressure exerted by each actuator on the respective brake disk.
To measure the aforesaid pressure, pressure sensors are used with high sensitivity both at low pressures and at high pressures, and likewise with a high full-scale value. In fact, there is particularly felt the need to measure pressure with a double measuring scale in order to measure both low pressures and high pressures with high precision. Furthermore, the force with which the pads are pressed against the disk may assume values from 0 N up to a maximum comprised in the range 10,000 to 35,000 N, according to the braking system.
There are currently known sensors capable of measuring high pressure values, which are made with a steel core, fixed on which are strain-gauge elements.
The strain-gauge elements detect the geometrical deformation of the core to which they are associated by variations of electrical resistance. However, these sensors, for reasons of reliability, size, and costs may be applied and used only for characterization and development of a braking system of the type described previously, but not in the production stage. Furthermore, they do not have a high precision and have only one measuring scale.
Likewise known are integrated pressure sensors, obtained with semiconductor technology. These sensors typically comprise a thin membrane suspended over a cavity made in a silicon body. Diffused within the membrane are piezoresistive elements connected together to form a Wheatstone bridge. When subjected to a pressure, the membrane undergoes deformation, causing a variation of resistance of the piezoresistive elements, and thus an unbalancing of the Wheatstone bridge. However, such sensors may not be used for measurement of high pressures, in so far as they have low full-scale values (namely, in the region of 10 kg/cm2), in particular considerably lower than the pressure values that are generated in the braking systems described previously.
A solution to the aforementioned problems is disclosed by U.S. Pat. No. 7,578,196, where, for measurement of high pressures, a membrane sensor is proposed provided with first piezoresistive elements, set in the proximity of the membrane, and second piezoresistive elements, set at a distance from the membrane, in a bulk area that is solid and compact. The first piezoresistive elements are designed to detect a deflection of the membrane that undergoes deformation under the action of low pressures, until a maximum deflection (saturation) is reached. The second piezoresistive elements are designed to detect a stress of a transverse type (but not longitudinal, in so far as there is no bending or phenomena of curving of the bulk area) that acts on the second piezoresistive elements as a result of an increase in pressure beyond the saturation pressure of the membrane.
This type of sensor provides a good accuracy of measurement at low pressures (signal supplied by the first piezoresistive elements), but a poor accuracy at high pressures (signal supplied by the second piezoresistive elements). Furthermore, this type of sensor does not discriminate between pressure variations lower than a minimum detection threshold.
For the feedback-control system of the braking system to function optimally, it is expedient also for the measurements made at high pressures to be accurate and sensitive to minimal pressure variations.